A new cyclosporine formulation has been developed by Abbott Laboratories to be bioequivalent to Neoral. The Abbott formulation has been tested extensively in healthy volunteers and in stable renal transplant recipients. A study in stable renal transplant recipients (six months post transplant) maintained on Neoral was conducted to compare the pharmacokinetics of cyclosporine during Neoral dosing and after converting to the Abbott cyclosporine formulation. Five twelve hour pharmacokinetics profiles of cyclosporine were obtained in each patient throughout the seven week period of the study. In addition, weekly trough levels were also collected throughout the study. The whole blood concentrations of cyclosporine were measured using an Abbott TDx Cyclosporine Monoclonal Whole Blood assay. Study results indicate that when the patients were switched from Neoral to Abbott cyclosporine, using the same dose, and back to Neoral the Cmax, Tmax and AUC of cyclosporine for the Abbott formulation are almost identical to the corresponding values obtained after administration of the Neoral formulation. The mean +-SD values (n=21) of cyclosporine trough blood levels were 213 +-80, 217 +-115, and 213 +- 77 mL on Study Days 1,7, and 14 during Neoral dosing. Days 15, 21, and 28 during Abbott cyclosporine dosing. The mean values of cyclosporine trough were 203 +-77 and 197 +-81ng/mL on Study days 29 and 35 after switching the patients back to Neoral. The Abbott formulation is well tolerated and no safety concerns have arisen during this ongoing clinical trial. Therefore pharmacokinetic studies indicate that he Abbott cyclosporine and Neoral formulations are fully interchangeable in stable renal transplant recipients.